A SWITCH for the better

Limit switches are sensors that provide feedback to keep some physical value (for example, pressure, temperature, or distance) within a preset range. Here we discuss mechanical limit switches and how they sense (and maintain) the position of machine parts. These electromechanical devices are triggered by physical contact, translating mechanical position into electrical responses. For example, on home appliances and automobiles they turn on lights when doors are opened. On manufacturing lines they sequence operations, limit the travel of machine parts, and detect conveyed items.

Mechanical limit switches always include some kind of actuator linked to electrical contacts. When an object runs into this actuator, it brings together or separates the contacts to make or break a connection. Usually this is an all-or-nothing deal; connections are either fully open or shut.

Mechanical limit switches do have moving parts that eventually wear out. Also, they must actually touch targets for output, which is inappropriate for some applications. However, limit switches are cost effective and extremely rugged. Joel Knutowski, product manager of proximity, limit, and cord-set sensing devices at Eaton Corp.'s Cutler-Hammer Sensor Products, Everett, Wash., explains, “Limit switches are a mature technology, but are still best in heavy-duty industrial applications. Too, many designers prefer them over inductive or photoelectric sensors because managers feel more comfortable with mechanical actuation. That's because their simplicity makes it easy for maintenance personnel to figure out how they work and how to install, troubleshoot, and replace them.”

Operation and options

Mechanical limit switches come in two basic forms: linear versions that use lever, leaf, slide, and plunger actuators, and rotary versions that use cam actuators. Each type moves differently when something runs into it. That's why actuators are often attached to operating heads that translate different motions into the motion actually needed to open or close the contacts. For example, leadscrew types use actuator nuts to trip stop contacts at both ends of travel. Despite varied implementations, all mechanical switches operate in the same basic way. Let's follow one through a cycle.

Picture a limit switch in its untriggered initial position. Its actuator contacts a target object and moves its pretravel distance.

Usually specified as a maximum value, pretravel is the distance from the actuator's free to operating position. Free position is usually measured from the switch mounting holes and specified as a maximum. There should be no force on the plunger at this point.

So at this instant, contacts are still in their normal untriggered position. When the actuator reaches its operating point, the contacts change from their normal to triggered position. In the case of a lever actuator, some overtravel allows the lever to move beyond the operating point. In contrast, overtravel distance on straight plunger actuators is a safety margin to avoid switch breakage.

Overtravel is the distance from the operating point to the end of plunger's travel; it is usually specified as a minimum value.

When force is finally released, the actuator begins the return to its initial position.

Release force is the force at which contacts resume their normal position; it is usually specified as a minimum value.

Though it's usually not specified explicitly, the release point is where the moving contacts also return to their normal, untriggered position.

Two contact styles

Mechanical limit switches have either snap-action or slow-breaking contacts. Slow-break contacts are moveable in a slide; they are forced directly to move with the actuator. Series made up of these slow-breakers fall into two categories. In units with break-before-make contacts, a normally closed contact opens before a normally open contact closes. (This allows the interruption of one function before continuation of another.) In switches with make-before-break contacts, the normally open contact closes before the normally closed contact opens. (This allows overlapping functioning, with the initiation of one function before the interruption of the first.) One drawback: Slow-breaking contacts do sustain arcing and reduce contact life.

Much more common are snap-action switches, assisted by a spring. When force is applied to the actuator in the travel direction, pressure builds in the snap spring until the actuator reaches the travel operating position. Then a set of moveable contacts accelerates from its normal position to a set of fixed contacts … and a signal is produced. If the force is removed, the actuator is released and the spring mechanism accelerates the moveable contact back to its original state.

The quick response of snap-action switches is useful for power switching, as well as slow and low-level signals (programmable controller inputs, for example) because they open and close regardless of actuator speed. Snap action also wipes contacts to effectively clean them.

Contact materials

Plain silver is typically used for applications requiring one to five-A ratings because of its excellent conductive properties. However, it is also susceptible to sulfide and oxide films. For this reason, gold alloy metal is used for lower-energy circuits — generally under one A. Silver cadmium oxide contacts are used for high-current applications, though the life of any switch is shortened at elevated levels. Platinum contacts are typically used in high temperatures.

Mechanical switches are slower than purely electronic sensors but they can switch high inductance loads, even to 10 A. They're also very precise in terms of accuracy and repeatability.

Actuation is probably to blame if a limit switch doesn't indicate position or control properly. Why? Limit switch failures are normally mechanical in nature. Since they have no internal electronics, mechanical switches do not emit electromagnetic noise — and are immune to such effects, even at extremely high levels. Contrast this with solid-state devices: They require continuous external monitoring to continuously verify their operational status. Knutowski explains, “With little to no solid-state components, mechanical switches are not affected by electrical noise and there is little mystery to the cause when they do happen to fail.”

Overtravel control: It's crucial to limit overtravel's force on the most delicate part involved — switch contacts. One method is to use external controls to keep the overtravel itself within basic switch specifications (usually 0.006 to 0.008 in.). Another straightforward means of controlling overtravel is with a physical stop on the switch.

Actuator position: When designing an external actuator mechanism, the amount of time the switch stays in its free position (vs the actuated position) is influential. If possible, switches should be wired so they're in the free position longer. This makes for longer switch life. If an application requires the switch be held in the operated overtraveled position, a “helper leaf” can be used under the switch actuator.

Actuating force: During basic switch operation, actuation force should always be applied perpendicular to the switch actuator. Another important value is differential — the distance between the operating and release points. It is usually limited by a maximum value.

Special cam considerations:Rotary limit switches should never act directly on switch actuators because this shortens switch life by distorting switches where direct interaction takes place. To prevent distortion, an auxiliary leaf can be used between the cam and switch actuator. This way, the leaf is subjected to side motion while the switch actuator is only subjected to vertical motion.

Two more tips: Cam rise for most applications should be around 45° and the dimension between high and low cam diameters should be the total distance from the free to the minimum overtravel position.

Special thanks to Paul Weir, Haydon Switch and Instrument Inc., Waterbury, Conn. For more information, call Haydon at (800)243-2715 or visitwww.hsiswitches.com.

Safety dance

One of the most important mechanical switch applications is personnel safety. Manufacturing cells often feature human operators working alongside robots, either serving up raw materials and components, or working in concert with them. To prevent any possibility of operator injury, safety standards require that robot controls be networked with operator-sensing devices such as safety light curtains, gate switches, and floor mats.

Mechanical safety limit switches on dynamic limiting devices increase work-cell productivity by allowing robots to continue production while guaranteeing operator safety should the robot controller software malfunction and send it into human-occupied areas. Balluff Inc. (Florence, Ky.) develops these mechanical limit-switch solutions with a number of manufacturers for use on robots.

When equipped with positive-opening contacts, they are adaptable to control-reliable circuitry that is single-point-fault tolerant per ANSI requirements. Henry Menke of Balluff Inc. explains, “Positive-opening designs guarantee electrical operation, even in the event that electrical overload causes internal contact welding.” Multiple-position limit switches, multi-channel cam drums, and complementary cam sets allow diverse redundant operation of up to three independent zones. Diverse complementary operation means that for 360° of rotation, two complementary angle cams — for example, 90° and 270° — operate two independent mechanical switches in a diverse and redundant manner.

Programmable units

The addition of a microprocessor can turn an ordinary limit switch into a programmable limit switch, opening the door to many new sensing and control features. Many programmable limit switches employ noncontact position-sensing transducers for position feedback. But as Knutowski explains, “Many new sensor technologies (optical, inductive, capacitive, and sonar, for example) are functionally dependent on the applications in which they're installed. Too, the roughness, color, angle, density, and material composition of the target can all affect the performance of these technologies — making their application difficult when they must work consistently over a required time period. On the other hand, heavy-duty mechanical limit switches are much more easily applied.” They can interface with many control-logic types including sinking, sourcing, and even TTL.

Whether mechanical or purely electrical, programmable switches usually work in tandem with a controller that compares sensed position to programmed setpoints — and determines if outputs should be on or off. This signal can be used to control anything from simple relays and logic devices to high-current solenoids and PLCs.

Gil Guajardo, product manager of safety products at Omron Electronics, LLC (Schaumburg, Ill.) explains, “When designers want to use safety switches with an existing relay, mechanical limit switches are most suitable, since non-contact types often require their own special safety relay.”